1. Field of the Invention
The present invention relates to a head assembly for recording information on an optical disc for reading out information already recorded.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional optical head assembly which is provided with a laser diode 104, a collimator lens 105, a beam splitter 106, an objective lens 107 and a photo detector 109. Laser beam from the laser diode 104 is collimated through the collimator lens 105, which collimated beam passes through the beam splitter 106 and is focused by the objective lens 107 to form a spot on a disc 113. Part of the incident beam is reflected by the disc 113, again passes through the objective lens 107, then its advancing direction is changed 90.degree. by the beam splitter 106 and thereafter a conversion is made into an electric signal by the photo detector 109. A differential amplifier 117 detects a difference in output between two sections in the photo detector.
Tracks (or guide grooves) 114 which comprises pits are formed on the disc 113. In order to record or reproduce information accurately, a beam spot must be positioned at the center of the track 114. In this connection, the optical head assembly in question is provided with a tracking sensor to detect a deviation of a beam spot from the center of the track 114.
Operation of the tracking sensor will now be explained. FIG. 2 shows a principle of tracking sensor called a push-pull method. As shown in FIG. 2(a), when a focused spot is positioned at the center of the track 114, a reflected laser beam is approximately bisected to form two reflected beams 115, which in turn pass through the objective lens 107 and the beam splitter 106 and form an image symmetric with respect to the center on the photo detector 109, as shown in FIG. 2(c). The photo detector 109 is divided into two sections 109a and 109b which are equal in characteristic and shape, as shown in FIG. 2(c). According to a method for realizing such characteristic, an effective surface of the photo detector 109 is divided in two by a symmetry axis. When a focused spot is positioned at the center of a pit, reflected beams irradiate the two sections of the photo detector equally, as shown in FIG. 2(c), so electrical outputs of the photo detector 109 are equal to each other as in FIG. 2(e). On the other hand, in the event a focused spot deviates from the center of a pit, the right and left of reflected beams become asymmetric as shown in FIG. 2(b), and also as to the image on the photo detector, the two sections are different in the quantity of light irradiated as shown in FIG. 2(d). Therefore, by taking the difference in electrical output between the two sections, there is obtained such a characteristic in which the axis of abscissa and that of ordinate represent a deviation of a focused spot from the pit center and a difference in electrical output, respectively.
In the event a central optical axis of the photo detector 109 itself deviates from an incident optical axis of the beam splitter 106, the image on the photo detector also shifts as shown in FIG. 2(f) and the difference in electrical output becomes like that shown in FIG. 2(g).
Thus, the conventional optical head assembly shown in FIG. 1 comprises the combination of such discrete parts as lenses, beam splitter and photo detector. In assembling, therefore, it has been necessary to make a fine adjustment of component mounting positions in order to align their optical axes accurately. According to a method for simplifying the construction of an optical system, a thin film dielectric light conducting layer, a converging type grating coupler and a thin film-like collimator lens are integrated on a single substrate. In this type of optical head assemblies, however, a tracking sensor function has not been considered yet.
Another conventional tracking system, as shown in FIG. 3, includes a diffraction grating 118 disposed between collimator lens 105 and beam splitter 106 and a sensor lens 119 disposed between beam splitter 106 and photo detector 109, in addition to the components shown in FIG. 1. The photo detector 109 used in this method comprises a first or central element for detecting a zero order beam and two side elements positioned on both sides thereof.
Laser beam emitted from the laser beam source 104 becomes a collimated beam, which in turn is diffracted by the diffraction grating 118.
In this type of device, usually a total of three diffracted beams--a zero order beam of a high intensity and two first order beams--are utilized. These three beams pass through the beam splitter 106 and are focused by the objective lens 107 to form three focused spots 114a, 114b and 114c on the track 114 of the recording medium 113.
The beams reflected by the track 114 pass through the objective lens 107, then are bent 90.degree. by the beam splitter 106, condensed by the sensor lens 119 and are incident on the photo detector 109.
In order to record or reproduce information accurately with respect to the track, the focused spot 114a of beam on the surface of the optical disc must be positioned at the center of the track 114 as shown in FIG. 4. To this end, optical head assemblies of this type are provided with a function (tracking sensor function) for detecting a deviation of the focused spot from the track center.
According to a tracking sensor method called three-beam method, as shown in FIG. 4, the focused spot 114a, of the zero order beam is located on a center line 120 of the track 114, while the two focused spots 114b and 114c of the first order beams are biased with respect to the center line 120 of the track 114.
Reflected beams from the three focused spots are detected each independently by the three elements of the photo detector 109. Then, a difference between output signals corresponding to the quantities of light of the two first order diffracted beams among three output signals from the photo detector 109 is detected by the differential amplifier 117 to obtain such an output as shown in FIG. 2(e). In this way, a deviation from the center of the focused spot of the zero order beam can be detected as an electric signal.
Thus, the conventional optical head assembly comprises the combination of discrete optical parts such as lens and beam splitter, so it has been necessary to use a fine adjustment mechanism in order to align the optical axes of those parts accurately in assembly. Particularly, in order to attain the tracking sensor function, it has been necessary to use a mechanism for rotating the diffraction grating.
Since the apparatus is constituted by such discrete parts and requires such fine adjustment mechanism, it becomes larger in size and the apparatus assembling and adjusting cost is increased.
To reduce the apparatus size and simplify its construction, there has been proposed a construction in which optical elements are integrated on a single substrate. However, the tracking sensor function has not been considered.